Orthogonal Frequency Division Multiplexing, also abbreviated to OFDM, will be the modulation technique of choice for future broadband wireless communication systems. In OFDM based systems, a user data stream is split into parallel streams of reduced rate. Each substream then modulates a separate subcarrier. By appropriately choosing a frequency spacing between subcarriers, the carriers are made orthogonal and some spectral overlap between the subchannels is permitted, leading to a better spectral efficiency than using simple frequency division. OFDM is especially attractive for high-speed wireless communication systems because it is robust against multi-path fading, intersymbol interference, and against narrowband interference. This promising modulation technique has already been accepted as physical layer for the Wireless Local Area Network (WLAN) standards IEEE 802.11a and ETSI Hiperlan-2. Both WLANs are operated in a 20 MHz wide radio channel in the 5 GHz frequency band, and provide user-selectable data rates between 6 and 54 Mb/s.
Before a user data packet consisting of several fixed length OFDM symbols can be exchanged over a radio channel, an OFDM receiver has to be adjusted so that it can successfully reconstruct the transmitted packet from the noisy, distorted signal received at an antenna. For this purpose, a preamble with known training symbols is transmitted prior to the user data packet over the radio channel. The preamble should allow the receiver to estimate a correct gain setting of a Variable Gain Amplifier (VGA) in the radio frontend, the frequency offset between the transmit and receive clock, and the OFDM symbol timing.
U.S. Pat. No. 5,732,113 is related to timing and frequency synchronization of OFDM signals. A method and apparatus for rapid timing synchronization, carrier frequency synchronization, and sampling rate synchronization of a receiver to an orthogonal frequency division multiplexed (OFDM) signal is disclosed. The method uses two OFDM training symbols to obtain synchronization. A first OFDM training symbol has only even-numbered sub-carriers, and substantially no odd-numbered sub-carriers, an arrangement that results in half-symbol symmetry. A second OFDM training symbol has even-numbered sub-carriers differentially modulated relative to those of the first OFDM training symbol by a predetermined sequence. Synchronization is achieved by computing metrics which utilize the unique properties of these two OFDM training symbols. Timing synchronization is determined by computing a timing metric which recognizes the half-symbol symmetry of the first OFDM training symbol. Carrier frequency offset estimation is performed in using the timing metric as well as a carrier frequency offset metric which peaks at the correct value of carrier frequency offset. Sampling rate offset estimation is performed by evaluating the slope of the locus of points of phase rotation due to sampling rate offset as a function of sub-carrier frequency number. However, the patent specification does not define a suitable gain control scheme and thorough receiver start-up for an IEEE 802.11a or ETSI Hiperlan-2 standard.
From the above it follows that there is still a need in the art for an improved method for adjusting of gain, frequency, and symbol timing in OFDM receivers. The adjustment of gain setting, receiver clock frequency, and symbol timing shall be achieved as quickly as possible in order to allow timely and reliable reception of user data.